1. Field of the Invention
The present invention relates to a noise detecting apparatus and an AM broadcast receiving apparatus that are capable of accurately detecting a short-period noise and effectively removing the short-period noise discharged from a streetcar, a power transmission line, etc.
2. Description of the Related Art
FIG. 6 shows a conventional configuration of an AM broadcast receiving apparatus 1 with a noise removing function. As shown in FIG. 6, the AM broadcast receiving apparatus 1 comprises an antenna 5 for receiving airwaves, an FE unit 10 for tuning and high frequency amplification, an IF unit 12 for frequency conversion and intermediate frequency amplification, a detecting unit 13 for AM detection of a received signal converted to an intermediate frequency signal, a noise detecting apparatus 14 for detecting presence of a noise and outputting a noise detection signal, a noise canceler 15 for removing the noise contained in received signal, an amplifying unit 16 for amplifying an audio signal, and a speaker 17 for outputting an audio.
FIG. 7 shows a configuration of the noise detecting apparatus 14. The noise detecting apparatus 14 comprises a filter 71 for limiting a band of a detected signal output from the detecting unit 13, a low-pass filter 72 for smoothing the signal output from the filter 71, a comparator 73 for comparing the signal output from the low-pass filter 72 with a predetermined threshold and outputting the noise detection signal when the level of the smoothed signal exceeds the threshold (see Japanese Patent Application Laid-Open Publication No. 2001-186031).
In the case of, for example, the AM broadcast receiving apparatus 1 installed in a car, to prevent deterioration of audio quality due to a pulsing noise of a comparatively short period (hereinafter, short-period noise) discharged from the streetcar, the power transmission line, etc., a mechanism to effectively remove the short-period noise is required.
However, the AM broadcast receiving apparatus 1 of the conventional configuration described above can not necessarily remove the short-period noise effectively. Namely, since the conventional noise detecting apparatus 14, as shown in FIG. 7, detects the presence of the noise based on a detected signal of a broad band output from the detecting unit 13, when the frequency of the short-period noise is in proximity to a noise attributable to an adjacent broadcasting station (hereinafter, adjacent disturbing wave), the noise canceler 15 is frequently caused to operate due to the adjacent disturbing wave, resulting in the deterioration of the audio quality of the AM broadcast receiving apparatus 1.